www.tomveatch.com / WIG

The WIG Boat Puzzle

A trajectory of curiosity

I've been interested in boats that use the Wing In Ground Effect for a couple decades. The low-energy soaring of pelicans near waves, the ground effect on a frisbee floating instead of landing after gliding down near to the ground, and my ground-school teacher explaining ground effect to cousin Rosanna and me in 2001, have inspired me to think about this subject. The Russians call it the "screen", or "ekran", as though there is a kind of screen along the surface of the ground or water, that lets you through and partly doesn't let you through, as you're landing.

Even before that, my friend Norbert Wu's father, who was an aerodynamics professor at Georgia Tech, had explained to me once about vortex generation under bird wings, and how commercial aviation is trying to figure out how to use that tremendous lifting power.

Driving home from ground school with Rosanna, I started thinking of designs of a boat with wings.

My first thoughts had water propellers, and a canard wing in front. My latest thoughts have obsessed over the Coanda effect and blown wing tops with controlled-aspect-ratio wings.

At every step it seems like the secret design is but a few steps away. Maybe that's the true take-away from this fascinating area, but meanwhile, it seems like we are just a few steps away from cracking the puzzle, and I love that. Please join me as I put some pieces on the table, and consider fitting them together in different ways.

Models to Follow

RCTestFlight: a WIG model

I love this guy's work: ground effect vehicle over snow and planing water vehicle. Can't find his actual name to give credit but his YouTube ID is RCTESTFLIGHT. You might say his experiments are primitive but I say they are brilliant and moving things in the right direction. First, it looks to me like a Coanda effect design. Second, in blowing onto the wing upper surface, his design rejects PAR, correctly in my opinion. Third, he has a wingtip flying in water contact with little detriment.

Edderitz: a fast boat

I'm also quite impressed by this example, I'll call it the 2015 Edderitz boat.

For high on-the-water speed, I like its wide-set front sponsons and rear drive. A balanced WIG boat should work well as a high-speed boat also, with stability and efficiency operating in full water contact.

A target

Consider as a goal for design a WIG boat with the abilities of the 2015 Edderitz boat, which keeps stably close to the surface. The Edderitz boat does a great job balancing the two forces of air pushing it down to the surface and water keeping it above the surface. A similar balance, even more difficult to achieve, would be between the non-lift at a higher elevation due to being out of ground effect, and the lift at a lower elevation due to ground effect. For each velocity there is some optimum, and if stability can be achieved, more than stability but a set of parameters where given the velocity then the elevation is specified and there are strong forces pushing both up and down to keep the vehicle at that elevation. That's the goal. The Edderitz boat at 1/8 to 1/2 wingspan elevation. How can we achieve that? Fly by wire? "Pre-tensioned" hydraulic actuators following a balance point? What are the forces, can we quantify them and design to float at the right points? That's what I'd like to see, and I offer the following puzzle pieces to help us all get there.

Puzzle Pieces

Let's start with the bad ideas, then the questionable ideas, then what seem to me to be the good ideas.

Bad Ideas

Death by center-of-lift variation.

Ok here are some ways to hopefully not die. If you are just designing a fast boat, then, like Edderitz,
  • make sponsons in front (or die thus = face-plant crash with sponsons not in front) and
  • make them aerodynamically neutral (or die thus - backflip crash with sponsons in front but not aerodynamically neutral).
In our case (WIG boats), the basic thing is that if your ground effect is from air stagnation under the wing, then the lift comes primarily at the trailing edge of the wing where all that air is finally maximally crushed under the trailing edge, and that pushes the vehicle up. The air at the front part isn't compressed yet, so it doesn't push upward that much. So stagnation effect lift is located emphatically rearward. Then as soon as the vehicle rises out of stagnation ground effect, the air isn't nearly as compressed at the trailing edge, and the whole wing contributes more equally to the total effect, irrespective of front or back, so the center of lift moves forward. Therefore once the vehicle catches some lift to get 1/4 or more of a wingspan up, or especially if in the back it gets up even just a little, then the relative compression at the trailing edge drops off dramatically. Compare the thin edge of air under a 5-percent elevation-to-span ratio scenario, for example, what will happen to the air compression ratio at the trailing edge if the wing rises by half a span? Instead of getting all the air crushed into 5% of the space at the very rear edge, it barely compresses at all under an edge with a 55% gap; the relative compression is much less between main undersurface wing area and trailing edge area. Thus a bit away from the ground, the extreme tail edge concentration of lift shifts toward the center of the wing chord. Then when the center of lift suddenly distributes forward to the center of the wing, because you were balanced on the rear-edge center of lift, now suddenly the whole thing flips up and backwards and you get this and this and this. Nice way to die, is what I'm saying. This is a problem.

PAR: Power Assisted Ram

No! Don't Blow It Up! I love it that RCTestFlight's design places the prop above the wing, because higher speed air is lower pressure so more lift up there. These designers with PAR (power assisted reinjection of air under the wing) are creating the wrong effect!

  • Thus I don't much like the jets blowing air under the wings for initial liftoff. Alexeev spent a lot of time on this one.
  • Doesn't fast air above slow air below represent the basic idea of lift on a curved-top airfoil? Bernoulli, anyone? Then why blow fast air underneath? It can only work in stagnation, when the rear edge of the wing is approximately underwater, then you're blowing up a very leaky balloon with a jet engine, sure it'll blow up a bit, but it'll spray water everywhere like crazy and it'll barely work. Mostly blowing between wing and water amounts to sucking the wing down to be close to the water. Delifting, not lifting. Lifting only at the barest beginning, getting the wings just out of the water. Maybe that's an argument for super low wings, though, as you see Alexeev's planes were pretty flat bottomed. But I am reminded of the old two-balloons experiment: blow between them (fast air) and the balloons bang together, bang bang bang bang, a little of the pressing apart happens, but mostly a lot of pulling them together happens. And then it's a repetitive banging thing. Which we don't want, neither the pulling two surfaces together with fast air rushing between them, nor the repetitive banging. So for me for now, let's scratch the PAR (power assisted ram) idea, okay? You wiser people, please persuade me otherwise. Meanwhile, let's forget it.
  • PAR is also unnecessary for the purpose of getting the wings out of the water if the design starts with wings already out of the water. Then normal propulsion can be used, and any hydrodynamic design will allow smooth acceleration up to the point of liftoff. For example, have a high wing on a shoulder blade, have a water prop pushing at low speeds and jointly with air propulsion during liftoff acceleration, then let it spin out and crank up out of the way after liftoff.
  • You might even want to force the beast to stay in water contact until past the point of minimum liftoff speed, in order to make use of the water propulsion up to a higher speed, then let it pop up out of the water, letting the water propulsion system shut down when there is enough power in the air propulsion system to keep it flying. What would that take, eh?

External take off power

  • Here's a moonshot inspired, otherwise bad, idea. The much-higher power requirement for take-off might be remedied by externally-applied acceleration: a slingshot launch method. Imagine a weak WIG-boat that can putt-putt along the water, or fly above it, but that can't make the transition without help. It would be safe but not convenient, because if you fall out of the sky you can still putt-putt along to wherever you need to go. Eventually. No, let's skip this idea, in service of self-rescue capability, and thus of general functionality. I'll call it a bad idea.

Bad names

Naming is a struggle for this category of most fascinating vehicles. It has the worst names: ekranoplan or screen-plane in Russian, WIG, AGEC, WISES, these names suck.

I don't have a solution, though I don't mind airboat or wingboat. Anyhow W-In-G sounds more like WING than WIG to me. Whatever, just use all the names in your web pages so everyone can find it, whatever you want to call it. I hate it that we're stuck with WIG Boat.

Not Obviously Bad Ideas

Canard guides underdraft

Early in thinking about WIG designs, it seemed to me that a small wing out front, a canard, could generate a downdraft below the main wing, producing greater pressure there and greater lift. (See WeberWig1 photos, also Fig. 10, photo of the Odessa Institute of Merchant Marine Engineers' WIG boat under Yu. A. Budnitsky.) If the problem of a WIG boat is packing some more airflow under the wing at a given speed, then a canard could help.

I have gotten some ridicule for this idea, but later I noticed that competition hydrofoil boats use a canard design. Between the front sponsons and the body is commonly a short wing with a gap behind it whereby air flowing over this first wing can get sucked under the following, longer-chord (body) wing or boat undersurface, and thereby give more of a cushion to ride on.

Also canards are seen in the "Japanese WISES" design here and the Jorg Tandem Wing design here. So it's not clearly a bad idea.

Water or air propeller?

Perhaps my first WIG idea was the intuition that a water prop might be more efficient than an air prop because it's pushing against an incompressible and dense material -- water -- which ought to be better for mechanical energy transfer than pushing against a compressible light material, air. Years later I found that N. I. Belavin on page 14 reports that about double the thrust comes from water props as compared with air.

A stupid idea here is this: WIG boats have a limited safe velocity range, so it might be an advantage if the power mechanism disengages when the boat flies up too high, and the prop comes out of the water. I don't think so: we want a design that provides actual control!

Finally, the efficiency of a high-speed boat prop depends on the blade being half in and half out of the water, a very very precise elevation requirement, which even medium waves will prevent. So my thinking has shifted to air propellers, which can be efficient too, and even their inefficiency in pulling in side channel air is a benefit in the Coanda effect much discussed here.

Bi-modality?

  • Consider a vehicle that handles equally with air propulsion, lift, and control surfaces and with water propulsion, lift, and control surfaces. Maybe two engines, two propellers, one for each medium. Maybe air wings AND hydrofoils, both. Maybe an air rudder like the giant Jorg tail (see here) AND a water rudder and front-corner sponsons. Effects in each medium should be coordinated, but can help each other during transitions, for example, the vehicle will be lifted by flotation then hydroplaning then aeroplaning with a weighted mixture of effects adding to the total results for lift, and similarly for propulsion and control.
    • Compromise designs can lose on both fronts, but I would like to see this for myself rather than give up on the basis of mere principle. Evidently specific distributions of wave height and separation should be the basis of design, and should be practically enforced as hard limits on actual operation, particularly as to take-off.
  • I mentioned water ruddering above. Air rudders have to be enormous to be effective: see Jorg. Maybe we do need some combination with a water tail rudder for directional control, perhaps with a fly-by-wire adjustment algorithm or other control system making the turn smooth irrespective of the moment-to-moment variation in the amount of rudder water contact. An Edderitz boat with poor directional control would be scary. But a tiny amount of water ruddering would resolve that issue completely. If it can be done safely, stably in potentially wave-filled environments, and without much drag. Big if's.
  • Considering much slower speeds, a likely bonus feature needed for widespread, consumer type use might be a water prop for in-the-water putt-putt movement, parking and getting out into the main ways, as in marinas and close-in boating areas where the giant winds of an air propellor might not be neighborly. Double the thrust for the same horsepower, sure why not. You might need a separate motor, or else some efficient means of transferring work from a front air prop to a rear water propeller (like a separate electric motor). Yes, if the boat can hike itself into the air it ought to have the propulsion to move around a marina, but we might also need to consider the neighbors part of the time.
  • Thus some form of bimodality may be needed.

Wing/body shape controls

  • I can imagine a few mechanisms for modifying wing/body parameters:
    • One is a "shoulder blade", a mechanism for raising the wing from low-wing (in WIG flight) to high-wing (in flotation). I don't like the wing stuck in the water during taxiing or docking, it should be high. Some rotating truss design should be able to lower the wing once airborn (to reduce wave battering on the body at a given wing elevation above the surface).
    • Another is a parking mechanism, rotating a single-span wing back, or folding two spans on each side against each other like a bird. The bird model is attractive if probably unrealistic, but should be understood: after landing, birds will fuss around and fold the wings under, up, and back until finally at a low-energy rest position. Upon takeoff, they sweep wings backward generating a horizontal-axis vortex with the top rolling toward the tail of the bird, followed by a forward sweep making use of the added lift produced by flying over a vortex that is rolling under your wing. This is Dr. Wu's point.
    • Many such adjustments such as folding multi-span wings could require quite manipulable wing shapes, and in turn the feather concept has its value, as feathers overlappingly form a conjoint surface of manipulable thickness and shape, each very light and controlled at the attachment end by shrugs and stretches in coordination with the whole group to form the wingform of current utility, depending on landing, lifting off, soaring, etc. Of course bats fly happily without feathers, depending on a skin stretch factor that is an alternative for us also in this design space.

Good Ideas

Sponsons

Water contact can be more than a drag. If wave height is normally distributed, then occasional freak waves are inevitable, and the design of WIG boats should allow for non-catastrophic water contact during normal flight. This means knifing sponsons in the front corners where the bounce off the water is pretty soft.

At the same time, sponsons must be aerodynamically neutral (or die). Kevin Curran's Lehigh University thesis on aerodynamics of high-speed sponsons has influenced my thinking: the front left and right water-contacting corners of a WIG boat should be a bit like Curran's Sponson A, that is, aerodynamically neutral in varying angles of attack, but hydrodynamically efficient, with a step to reduce wetted area at speed.

  • I take it as proven that high speed water-contacting vehicle designs including WIG boats must have front right and left corner sponsons which are aerodynamically neutral while providing hydrodynamic lift.
  • I take it as desireable that sponsons provide mixed-medium stabilization with smooth rather than abrupt impact from free flight at some range of angles of entry. Therefore considered from bottom upwards, they should begin quite small and thin in displacement volume and in wetted surface area, perhaps knife-like vertical or curving hydrofoils designed to impact the water during flight creating only minimal drag and medium lift, at low wave penetration heights. Their water displacement volume should remain small up to fairly deep penetrations as through wave tops or to stabilize flight when rolling onto one side or the other, and get larger in displacement only when the sponson is operating as an actual float.

Flying with a wing in water contact

I think a WIG boat should be comfortable in mixed media. That is, not just putt-putting in water and flying in air, but flying in more or less simultaneous contact with the water. Why? Because practical WIG use will certainly have to deal with the transition, in which both flight and water contact occur, and if that transition is a happy stable state, then good, whereas if not, then we have danger and instability.

RCTestFlight's vehicle shows success with at least one wing corner in water contact. I'd like to see both as much as neither. It likes to fly in a large curve with a single corner in water contact. For increased stability, I say, turn the corner sponson/pontoon into a knife edge so it has minimum drag, tweak it so it tracks straight, if possible, with either one or both front corners in water contact during normal cruising flight speeds.

What Edwin Van Opstal said.

  • Comparing the two sources of ground effect, span-dominated versus chord-dominated, the main effect is span-dominated. Van Opstal's graph shows that at a height of 5% of wingspan, the drag is reduced by to a 30% fraction compared with free flight. Whereas at a height of 5% of the chord, the drag reduces only from 1.1 to 0.8 as compared with free flight. 5% of the CHORD! It would require a chord longer than the average wingspan to get far less than half the effect, if my reasoning is correct. Therefore ground effect is hugely dominated by the wing span effect. Another way to say this is that wing downwash is a bigger energy sink than wingtip vortices. And therefore making a long-chord wing, which many designs are based on including Lippisch and Jorg, is a mistake; whereas a short-chord, longer span wing, is the way to maximize ground effect.

    Witness low-soaring birds such as seagulls and pelicans: Long narrow wings.

    If the Alexeyev ekranoplan designs have short wings, it's because the lift is so great they don't need more.

From Lippisch to Coanda

Consider a sequence of proposals taking off one from the other:
  • Lippisch by his designs said, spread out the rear edge in a front-back dimension by making the rear edge sharply diagonal forward as the wing reaches farther out, so that a good part of the "rear" edge is quite a bit forward of the rearmost part of the rear edge. He also added a small pontoon wing also up front, to add more front-loaded lift to the mix. Then the change will be less when the nose kicks up.

  • But following Van Opstal's findings, consider de-emphasizing the stagnation (chord-dominated) ground effect, and instead emphasizing span-dominated ground effect lift (where ground effect efficiency comes from reduced wing downwash in proximity to the ground surface) or plain lift or a blown wing top in your design.

  • A compromise shape might use BOTH a blown wing top with prop above and long chord along the centerline of the vehicle body -- AND at the same time a long span. Think Lippisch X-112 without the delta shape, just a long chord for a middle section of wing, while emphasizing extra long pontoon wings.

  • In idle imagination I wonder about a biplane, with Coanda lift on an upper, long-chord, short-span wing, perhaps containing the passenger/cargo spaces within the wing itself, all above a lower, long span wing using ground effect lift. Was this connected to the thinking behind original (1920's) biplane designs with a shorter lower wing and longer upper wing? When the prop blows harder, the plane will lift hard and could even rotate forward like an accelerating helicopter. With reduced prop power, ground effect on the lower wing still keeps you floating along.

  • In excited imagination I conceive a wing of variable, adjustable, controlled aspect ratio.
    • In pancake configuration, it lifts strongly, artificially, at low velocity with a long chord, blown wingtop.
    • In pelican configuration, it lifts strongly, naturally, with a short chord, long span, efficiently propellored wing with little or no Coanda blown wingtop.
    • In mixed configuration, wing tips ("fingers"), extend before wing inner sections ("elbows"); doing so they grab more lift as velocity and still-air bernoulli effect permit; as they begin to bite, more of the wing tips can extend, eventually sacrificing Coanda-captured surface area and the lift therefrom,as strong and reliable pelican lift safely permits.
    • Propellor downwash orientation can be maintained so that lift can be quickly reasserted by wing retraction into Coanda (downwash, captureable by wing entering within the) space.
    • Possibly, if safe, prop orientation may be straightened, up, closer to path-parallel, when even over the central body, Coanda lift is no longer needed.
    • Cellularly connected, adjustable x/y/z ratio, jointly but not identically controlled, wing structural units, may be jointly designed to achieve arbitrary end configurations and transitional shape patterns under one or few control variables, similar to morphing. Cells may have constant volume or contents, etc, but be flexible (in at least certain designed dimensions) surfaces; I hate to suggest rubber since that limits temperatures of operation. Cell shape adjustments may be via motorized, hydraulic, or pneumatic power delivery and by central or distributed controls. Cells may also carry wing surface forming features (e.g., feather surfaces) to influence shared wing shaping and to maximize lift in the then current configuration.

Coanda effect lift

  • I believe the excellent lift and easy-liftoff qualities of RCTestFlight's vehicle are due to the Coanda effect whereby the outflow of the propeller above the wing entrains onto the upper surface of the wing, hence, due to the Bernoulli effect, provides extreme lift to the wing in addition to forward propulsion.
  • In his videos, 1 and 2, you can see the propeller axis points somewhat downward toward the top of the wing surface, thus making it easier for the Coanda entrainment to occur.
  • RCTestFlight's vehicle's wing has an extremely long chord. Why? In Coanda/Bernoulli lift enhancement the propeller outwash entrains onto the wing upper surface. The total lift force is proportional to the amount of wing surface to which the resulting lowered pressure applies. So you could increase lift by increasing the upper surface area impacted by the outwash, that is essentially by either increasing width (span) or length (chord) of the wing.
    • Increase the wing span, and nothing happens, because the outwash is centered on the propeller and can only spread laterally a limited amount. You could increase the amount of the span which is affected by using multiple propellers, or by providing ducts or vanes to spread the propulsion wash laterally across the wing. Ugh. Or instead...
    • Increase the wing chord (increasing the duration of airflow contact with the wing), and you directly increase the area effected by Coanda/Bernoulli. Thus more lift. The effect occurs over a larger area with a longer above-wing wash entrainment surface. It lowers the pressure above the wing over a larger total surface area.
    • So the outwash is essentially a narrow (though widening) but long resource for imposing lift onto a wing. Actually its shape is likely somewhat triangular, with greater span influenced as the wash reaches along a greater distance of the chord.
    For more, read about Blown flaps, especially look for the phrase Upper Surface Blow, which in the one built case, the YC-14, increased the coefficient of lift to 7, (compare with a Boeing 747 at high altitude cruise having a coefficient of lift of 0.52 according to this reference).
All this is consistent with Coanda lift as being responsible for the wonderful out-of-water lift and stable flight properties of RCTestFlight's vehicle, and a key puzzle piece in the search for the WIG boat of the future.

Coanda 2: Blown wingtop observations and discussion

  • So far, a blown wingtop design seems quite promising. Make the prop wash entrain to the top of the wing via the Coanda effect. Then it should will produce huge lift, according to the Bernoulli effect. 14x maybe. Shouldn't that be of some assistance especially during take off? This seems to have been part of Lippisch's thinking in the X-112, which has a propellor located so most of its outwash is above the wing. Yet the X-113 and X-114 which should be advances have the motor rearward and higher, so little wingtop entrainment and Coanda/Bernoulli lift can occur. Noticeably the X-112 seems to spend a lot more time at >1/5 wingspan elevations where the later models seem to fly lower, tighter to the water -- that is, reliant more on chord-based ground effect lift. The X-112 flies away from the water comfortably.
    • I just had an idea. Suppose you could vastly increase the chord of the wing to provide a larger blown surface and more lift during take-off, but at the same time vastly decrease the chord of the wing to spend more of the propellor energy on propulsion instead of lift. Overlapping "feathers" (slidable overlapping wing surfaces) on a moving understructure, to shorten and lengthen the chord. The biplane mentioned herein is an alternative. Lippisch's pontoon wings added to a stagnation ground effect reverse delta wing is also a way to deliver both effects in one vehicle.
  • The blown wing top concept is supported by the Custer Channelwing, which increases wing-top air velocity by creating a venturi effect within a half-channel over the wing, and demonstrates vertical take off and 8-13 lbs lift per horsepower.
  • The blown wing top concept is also consistent with what my impressions of watching the library of Youtube videos on WIG boats. Some designs seem to fly at 1/5 to 1/2 wingspan and can lift out of ground effect, though laboriously; I will call these the "floaters". Other designs fly with the training wing edges barely clearing the water, say 1/20 to 1/10 wingspan, and these cannot lift out of ground effect; I will call these the "huggers". The floaters include Lippisch X-112, Japanese WISES, and RCTestFlight. The huggers include Lippisch X-113/114 and derived forms, and Jorg. Huggers might be able to float if they get substantial extension wings outside their pontoon/sponsons, which give much more lift. Floaters also have blown wing tops (with the propellor outwash all or primarily on top of the wing), while Huggers seem to blow under the wings. Am I making my point?

Practical data

Materials

  • For hand-made RC prototypes: Blue or pink insulation foam. EPP foam. 6mm Devron foam. 1/2-inch wood dowels (square). Grayson Hobby Super mega Jet electric motor.80Amp ESC. Nanotech 3S lipopack 2200. or 1300mAh 3 cell battery. Glued receiver for waterproofing. Servos to turn control surfaces (rudder). Blue Wonder motors. Radio controller: DSM2 DX7 Model Match[TM] technology. 3S brushless motor with watercooling for high speed boats.
  • For full size systems: aluminum boat building technology.

Dimension take-offs

RCTestFlight's snow-sled flies nicely in ground effect with estimated dimensions 1: rear vane height and length; 3 sled length; 2.5 sled width; 0.8: propeller axis elevation above sled frame; 2: distance from sled tail to delta wing front root; .75: distance from sled tail to delta wing front outside corner. 1.25 width of delta wing, each side. Rear pitch-control surface: full width, 0.5 length. 2.5/12 slope of propeller axis. He has end pontoons, full length,

Conclusion

Thank you for your interest and patience with this inventory of partially digested and integrated concepts. I invite you to share with me your thoughts, questions, corrections and friendly suggestions. And consider, if you are interested in this area, how might we help each other advance the state of this art?
        
 

Copyright © 2000-2018, Thomas C. Veatch. All rights reserved.
Modified: June 24, 2017, August & September 2018